You can test it on the paint somewhere inconspicuous. The alcohol should completely evaporate in short order, and it will take the water with it. Meanwhile, it will prevent freezing.

There is an o-ring inside that's supposed to keep out moisture, perhaps it's perished, or perhaps it's just a fluke. But water is definitely in the mechanism. Refer to my write up on my 190 locks, which are likely very similar in construction:

http://www.coolcatcorp.com/Mercedes%20Locks/Door%20Lock/190%20Door%20Locks.html